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Atomic number (Z), mass number (A) and isotopes of an element.
Chemistry | GRADE 11 | QCAA Board
Below are comprehensive study notes for the topic:
Atomic Number, Mass Number and Isotopes
1. Introduction
Atoms of elements are characterised by specific numbers of protons, neutrons, and electrons.
Three important terms used to describe atoms are:
• Atomic Number (Z)
• Mass Number (A)
• Isotopes
Understanding these concepts helps explain element identity, atomic mass, and variation among atoms of the same element.
2. Atomic Number (Z)
Definition
The atomic number (Z) is defined as the number of protons present in the nucleus of an atom.
\( Z = \text{Number of protons} \)
Key Characteristics
Determines the identity of an element
Each element has a unique atomic number.Equal to number of electrons in a neutral atom
\( \text{Electrons} = Z \)
Determines the position of an element in the periodic table
Examples
| Element | Atomic Number (Z) | Protons | Electrons |
|---|---|---|---|
| Hydrogen | 1 | 1 | 1 |
| Carbon | 6 | 6 | 6 |
| Oxygen | 8 | 8 | 8 |
| Sodium | 11 | 11 | 11 |
Example:
For Carbon
\( Z = 6 \)
Therefore:
• Protons = 6
• Electrons = 6 (in a neutral atom)
3. Mass Number (A)
Definition
The mass number (A) is the total number of protons and neutrons present in the nucleus.
\( A = \text{Number of protons} + \text{Number of neutrons} \)
or
\( A = p + n \)
Determining the Number of Neutrons
The number of neutrons can be calculated as:
\( n = A - Z \)
Example
For Carbon-12
\(A = 12 \)
\(Z = 6 \)
\(n = 12 - 6 = 6 \)
Thus:
| Particle | Number |
|---|---|
| Protons | 6 |
| Neutrons | 6 |
| Electrons | 6 |
Nuclear Notation
Atoms are often written using the notation:
\( ^{A}_{Z}X \)
Where:
• (X) = symbol of the element
• (A) = mass number
• (Z) = atomic number
Example:
Carbon-12
\( ^{12}_{6}C \)
4. Isotopes
Definition
Isotopes are atoms of the same element that have the same atomic number but different mass numbers.
This means:
• Same number of protons
• Different number of neutrons
Example: Isotopes of Carbon
| Isotope | Atomic Number (Z) | Mass Number (A) | Neutrons |
|---|---|---|---|
| Carbon-12 | 6 | 12 | 6 |
| Carbon-13 | 6 | 13 | 7 |
| Carbon-14 | 6 | 14 | 8 |
All three atoms:
• Have 6 protons
• Belong to the same element (carbon)
But they differ in neutron numbers.
Example: Isotopes of Hydrogen
Hydrogen has three common isotopes.
| Isotope | Symbol | Protons | Neutrons |
|---|---|---|---|
| Protium | \( ^1_1H \) | 1 | 0 |
| Deuterium | \( ^2_1H \) | 1 | 1 |
| Tritium | \( ^3_1H \) | 1 | 2 |
5. Properties of Isotopes
1. Chemical Properties
Isotopes have almost identical chemical properties because:
• Chemical behaviour depends on electrons
• Isotopes have the same number of electrons
2. Physical Properties
Isotopes may have different physical properties, such as:
• atomic mass
• density
• melting and boiling points
because they contain different numbers of neutrons.
6. Relative Atomic Mass
Most elements occur naturally as a mixture of isotopes.
The relative atomic mass (Ar) is the weighted average mass of all naturally occurring isotopes.
Example:
Chlorine has two isotopes:
| Isotope | Abundance |
|---|---|
| Cl-35 | 75% |
| Cl-37 | 25% |
Weighted average:
\( Ar = (35 \times 0.75) + (37 \times 0.25) \)
\( Ar = 35.5 \)
That is why the atomic mass of chlorine is 35.5.
7. Uses of Isotopes
Isotopes are widely used in science and technology.
Medicine
Radioisotopes used for:
• cancer treatment
• medical imaging
Example:
• Iodine-131 → thyroid treatment
Archaeology
• Carbon-14 dating used to determine age of fossils.
Industry
• Detecting leaks in pipelines
• Thickness measurement of materials
Agriculture
• Studying nutrient absorption in plants.
8. Differences Between Atomic Number, Mass Number and Isotopes
| Feature | Atomic Number (Z) | Mass Number (A) | Isotopes |
|---|---|---|---|
| Definition | Number of protons | Total protons + neutrons | Atoms of same element with different neutron numbers |
| Determines | Element identity | Total nucleons | Variation of atoms |
| Symbol | Z | A | Same Z, different A |
| Example | Carbon = 6 | Carbon-12 = 12 | C-12, C-13, C-14 |
9. Key Summary
• Atomic Number (Z) = number of protons in the nucleus.
• Mass Number (A) = total number of protons and neutrons.
• Neutrons = A − Z.
• Isotopes are atoms of the same element with same Z but different A.
• Isotopes have similar chemical properties but different masses.
Applying Nuclear Symbol Notation \( ^A_ZM \)
1. Introduction
Atoms are often represented using nuclear symbol notation, which provides important information about the structure of an atom, ion, or isotope.
The general format is:
\( ^{A}_{Z}M \)
Where:
| Symbol | Meaning |
|---|---|
| (M) | Chemical symbol of the element |
| (A) | Mass number (protons + neutrons) |
| (Z) | Atomic number (number of protons) |
This notation allows us to determine:
• Number of protons
• Number of neutrons
• Number of electrons
for atoms, ions, and isotopes.
2. Understanding Each Component
Atomic Number (Z)
The atomic number represents the number of protons in the nucleus.
\( Z = \text{number of protons} \)
In a neutral atom:
\( \text{Number of electrons} = Z \)
Mass Number (A)
The mass number is the total number of nucleons in the nucleus.
\( A = \text{protons} + \text{neutrons} \)
Thus:
\( \text{Neutrons} = A - Z \)
3. Determining Subatomic Particles in a Neutral Atom
For neutral atoms:
| Particle | Formula |
|---|---|
| Protons | (Z) |
| Electrons | (Z) |
| Neutrons | (A - Z) |
Example 1
Determine the number of protons, neutrons and electrons in:
\( ^{23}_{11}Na \)
Step 1: Identify values
\( A = 23 \)
\( Z = 11 \)
Step 2: Calculate particles
Protons
\( = Z = 11 \)
Electrons
\( = 11 \)
Neutrons
\( = A - Z \)
\( = 23 - 11 = 12 \)
Result:
| Particle | Number |
|---|---|
| Protons | 11 |
| Neutrons | 12 |
| Electrons | 11 |
4. Determining Particles in Ions
Ions are atoms that have gained or lost electrons.
Two types:
| Ion Type | Meaning |
|---|---|
| Cation | Positive ion (lost electrons) |
| Anion | Negative ion (gained electrons) |
Important rule:
Protons never change during ion formation.
Electron Calculation for Ions
For an ion:
\( \text{Electrons} = Z - \text{positive charge} \)
or
\( \text{Electrons} = Z + \text{negative charge} \)
Example 2: Sodium Ion
\( ^{23}_{11}Na^+ \)
Step 1: Protons
\( = Z = 11 \)
Step 2: Neutrons
\( = A - Z \)
\( = 23 - 11 = 12 \)
Step 3: Electrons
Since charge = +1
\( \text{Electrons} = 11 - 1 \)
\( = 10 \)
Result:
| Particle | Number |
|---|---|
| Protons | 11 |
| Neutrons | 12 |
| Electrons | 10 |
Example 3: Oxide Ion
\( ^{16}_{8}O^{2-} \)
Step 1: Protons
\( = Z = 8 \)
Step 2: Neutrons
\( = 16 - 8 = 8 \)
Step 3: Electrons
\( 8 + 2 = 10 \)
Result:
| Particle | Number |
|---|---|
| Protons | 8 |
| Neutrons | 8 |
| Electrons | 10 |
5. Determining Particles in Isotopes
Isotopes are atoms of the same element with:
• same atomic number
• different mass numbers
This means:
• same number of protons
• different number of neutrons
Example: Carbon Isotopes
Carbon-12 : \( ^{12}_{6}C \)
Protons: | Electrons: | Neutrons: |
|---|---|---|
\( 6 \) | \( 6 \) | \( 12 - 6 = 6 \) |
Carbon-14
\( ^{14}_{6}C \)
Protons:
\( 6 \)
Electrons:
\( 6 \)
Neutrons:
\( 14 - 6 = 8 \)
Difference:
Only neutron number changes.
6. Worked Examples
Example 1
Determine particles in:
\( ^{27}_{13}Al \)
Protons
\( 13 \)
Electrons
\( 13 \)
Neutrons
\( 27 - 13 = 14 \)
Example 2
Determine particles in:
\( ^{35}_{17}Cl^- \)
Protons
\( 17 \)
Neutrons
\( 35 - 17 = 18 \)
Electrons
\( 17 + 1 = 18 \)
Example 3
Determine particles in:
\( ^{40}_{20}Ca^{2+} \)
Protons
\( 20 \)
Neutrons
\( 40 - 20 = 20 \)
Electrons
\( 20 - 2 = 18 \)
7. Quick Method (Exam Shortcut)
From nuclear notation \( ^A_ZM^{charge} \):
1️⃣ Protons | 2️⃣ Neutrons | 3️⃣ Electrons |
|---|---|---|
\( p = Z \) |
\( n = A - Z \) |
Neutral atom: \( e = Z \) Ion: \( e = Z^- (\text{positive charge}) \) \( e = Z^+ (\text{negative charge}) \) |
8. Summary
• Nuclear notation is written as:
\( ^{A}_{Z}M \)
• (Z) gives the number of protons.
• (A) gives total nucleons (protons + neutrons).
• Neutrons = A − Z.
• In neutral atoms: electrons = protons.
• In ions: electrons change according to charge.
• Isotopes have same Z but different A.
This notation is essential for understanding atomic structure, isotopes, and ionic species.
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